This invention relates generally to television signal transmission systems and specifically to a novel television transmission system and method having reduced ghost visibility.
An ever-present problem of television signal transmission and receiving systems has been that of ghost video images, simply referred to as ghosts. A video ghost or ghost image is developed by an electrical signal that is identical to the main or real signal, but displaced in time. The ghost signal travels a longer path and thus has a longer propagation time from the transmitter to the particular receiver, as compared with the real signal. Generally, ghost signals are created by reflection from objects that lengthen their transmission paths and it is not uncommon to encounter multiple ghost signals. Video ghosts are very often disconcerting to a viewer since the ghosts are lower intensity images that are displaced from the real images. The amount of the displacement is a function of the transmission delay between the real and the ghost electrical signal and the ghost signal intensity is a function of signal amplitude. Consequently, ghost images are variable and depend upon a number of factors, including the position of the transmitter, the location of the receiver and the terrain between the transmitter and the receiver.
The prior art reveals many attempts to cancel or minimize ghost images in television receivers. The major difficulty in developing a "ghost cancellation" system is that every transmitter and receiver combination presents a unique problem. Further, the problem may not remain static, that is, it may change depending upon changes in the physical environment in which the transmitter and receiver are located. For example, a particular television receiver installation may have a system installed for perfectly cancelling ghost signals from a particular transmitter. Yet, the later erection of a tall building that disturbs either the real or the ghost signal transmission path (or both) between the transmitter and the receiver will disrupt the ghost cancelling system.
As mentioned, the video ghost appears as a displaced image of lower intensity, depending upon its signal strength. If the electrical ghost signal achieves a level that approaches 25-30% of the real signal level, the receiver synchronization circuits will be unable to reliably discriminate between the two signals and no viewable picture will be possible. Consequently, not only are ghost signals disturbing in terms of viewer display, they can render viewing impossible.
The system of the invention does not cancel ghost signals. It does, however, minimize the visibility of ghost signals by reducing their amplitude by an additional 50% with respect to the real signal. With the invention, the real video image is flanked by a pair of symmetrical, reduced visibility ghost images. The symmetrically disposed ghost images of lesser visibility actually enhance the video presentation in many instances since they help to define the true center of the real image. In alphanumeric displays, for example, conventional ghost images are most objectionable because the eye has difficulty in distinguishing edges in the presence of strong ghost images. The symmetrical disposition on opposite sides of the real image of two ghost images of lesser but equal visibility helps the eye to concentrate on and to define the edges of the real image. Thus with the invention, displays of textual material become much more legible.